This invention relates generally to wireless communication systems, and more particularly to a technique for providing variable data rate connections over digitally encoded radio channels.
The first generation of personal wireless communication devices, such as cellular radio telephones, operated by allocating distinct individual radio carrier frequencies to each user. For example, in an Advanced Mobile Phone Service (AMPS) type cellular mobile telephone, two 30 kiloHertz (kHz) bandwidth channels are allocated to support full duplex audio communication between each subscriber unit and a base station. The signals within each such channel are modulated using analog techniques such as frequency modulation (FM).
Later generation systems make use of digital modulation techniques in order to allow multiple users to access the same frequency spectrum at the same time. These techniques ostensibly increase system capacity for a given available radio bandwidth. The technique which has emerged as the most popular within the United States is a type of Code Division Multiple Access (CDMA). With CDMA, each traffic signal is first encoded with the pseudorandom (PN) code sequence at the transmitter. The receivers include equipment to perform a PN decoding function in such a way that signals encoded with different PN code sequences or with different code phases can be separated from one another. Because PN codes in and of themselves do not provide perfect separation of the channels, certain systems have an additional layer of coding referred to as xe2x80x9corthogonal codesxe2x80x9d in order to reduce interference between channels.
In order for the PN and orthogonal code properties to operate properly at a receiver, certain other design considerations must be taken into account. For signals traveling in a reverse link direction, that is, from a mobile unit back to a central base station, power levels must be carefully controlled. In particular, the orthogonal properties of the codes are optimized for the situation where individual signals arrive at the receiver with approximately the same power level. If they do not, channel interference increases.
The forward link direction presents a different problem. In particular, a signal traveling from the base station to a subscriber unit may interfere with another signal in an unpredictable way as a result of the so-called near far problem. For example, faraway mobile units require relatively high power in order to be detected properly whereas close-in mobile units require lower power. The stronger signals may interfere with proper operation of mobile units located closer to the base station which typically operate with lower power levels. Unfortunately, this behavior depends upon the specific operating environment of the mobile communications system, including the topology of the surrounding geography, the juxtaposition of the subscriber units with respect to one another, and other factors.
In the past, it has been possible to set power levels individually to optimize each forward link channel so that interference is minimized. In particular, it has been suggested that each power level can be adjusted to affect an optimum received power level at the subscriber unit which tends to minimize interference.
In addition, coding algorithms such as forward error correction (FEC) type algorithms using convolutional, Reed-Solomon, and other types of codes, may be used to increase effective signal-to-noise ratio at the receiver. While such codes do provide increased performance in terms of lower bit error rates in noisy environments, by themselves they do not improve the difficulties associated with co-channel interference.
The present invention provides an additional degree of freedom by permitting individual traffic channel data rates to adapt to specific channel conditions. In particular, a forward error correction (FEC) coding rate may be adapted for individual channels. At the same time, a fixed number of FEC symbols is maintained per transmitted frame, independent of the FEC coding rates and power levels. This allows a different FEC rate or even a different FEC code to be assigned to each user channel, depending upon channel conditions, without changing the effective transmitted power levels.
For example, if the channel is experiencing relatively good propagation conditions, the FEC coding rate may be reduced and the number of input bits per FEC frame may be increased without changing transmit power levels. Because the overall information rate then depends upon the ratio of the raw data rate divided by the code rate, a higher information rate is obtained without producing greater interference to other user channels.
On the other hand, if a particular channel is in a relatively bad or marginal transmission environment, other steps can be taken to reduce the overall information rate. Specifically, rather than increasing the power level of the transmission, the effective FEC coding rate can be increased, and the number of input bits per FEC frame reduced. This then permits the channel to be more robust without increasing the transmit power level.
In a preferred embodiment, the FEC coding rate is changed by periodically sending a message to the intended receiver which indicates the coding rate to be used in future transmissions on each given channel. For example, in a typical implementation, a rate message may be sent on the forward link paging channel or sync channel directed to a particular receiver.
There are several advantages to the present invention. In a Code Division Multiple Access (CDMA) system, especially in environments where multipath fading or other poor channel conditions exist, power levels need not be adjusted in order to optimize the overall system information rate.